Fix info mem command for 32 bits host/64 bits target

Fix info mem command for 32 bits host/64 bits target

When running gdb on 32 bits host for 64 bits target, info mem command
truncates the target address to 32 bits, like in the example below
	(gdb) set architecture powerpc:common64
	(gdb) mem 0x100000000 0x200000000 rw
	(gdb) info mem
	1   y  	0x0000000000000000 0x0000000000000000 rw nocache

gdb/ChangeLog:

        PR gdb/15684
        * memattr.c (mem_info_command): Remove "unsigned long" casts.

Signed-off-by: Catalin Udma <catalin.udma@freescale.com>